Accurately modeling both the shape and color shade of a tooth are important functions for providing restorative dentistry and related services. Conventionally, the functions of determining tooth contour and matching tooth shade have been performed as separate operations. This makes it difficult to register or correlate the shade information to the shape information. Color shade decisions in particular are subject to human error and the overall accuracy often depends on the relative experience of the practitioner.
With the advent of digital imaging technologies, a number of tools have been made available for obtaining either surface contour data or color shade information from the tooth. One technology that has been adapted for obtaining tooth contour is fringe projection imaging. Fringe projection imaging uses patterned or structured light to obtain surface contour information for structures of various types. In fringe projection imaging, a pattern of lines of an interference fringe or grating is projected toward the surface of an object from a given direction. The projected pattern from the surface is then viewed from another direction as a contour image, taking advantage of triangulation in order to analyze surface information based on the appearance of contour lines. Phase shifting, in which the projected pattern is incrementally spatially shifted for obtaining additional measurements at the new locations, is typically applied as part of fringe projection imaging, used in order to complete the contour mapping of the surface and to increase overall resolution in the contour image.
Fringe projection imaging has been used for surface contour imaging of solid, highly opaque objects and has been used for imaging the surface contours for some portions of the human body and for obtaining detailed data about skin structure. However, a number of technical obstacles have prevented effective use of fringe projection imaging of the tooth. One challenge with dental surface imaging relates to tooth translucency. Translucent or semi-translucent materials in general are known to be particularly troublesome for fringe projection imaging. Subsurface scattering in translucent structures can reduce the overall signal-to-noise (S/N) ratio and shift the light intensity, causing inaccurate height data. Another challenge relates to high levels of reflection for various tooth surfaces. Highly reflective materials, particularly hollowed reflective structures, can effectively reduce the dynamic range of this type of imaging.
In fringe projection imaging overall, contrast is typically poor, with noise as a significant factor. To improve contrast, some fringe projection imaging systems take measures to reduce the amount of noise in the contour image. In general, for accurate surface geometry measurement using fringe imaging techniques, it is desired to obtain the light that is directly reflected from the surface of a structure under test and to reject light that is reflected from material or structures that lie beneath the surface. This is an approach for 3D surface scanning of translucent objects.
From an optics perspective, the structure of the tooth itself presents a number of additional challenges for fringe projection imaging. As noted earlier, light penetrating beneath the surface of the tooth tends to undergo significant scattering within the translucent tooth material. Moreover, reflection from opaque features beneath the tooth surface can also occur, adding noise that degrades the sensed signal and thus further complicating the task of tooth surface analysis.
One corrective measure that has been attempted to make fringe projection workable for contour imaging of the tooth is application of a coating that changes the reflective characteristics of the tooth surface itself. Here, to compensate for problems caused by the relative translucence of the tooth, a number of conventional tooth contour imaging systems apply a paint or reflective powder to the tooth surface prior to surface contour imaging. For the purposes of fringe projection imaging, this added step enhances the opacity of the tooth and reduces or eliminates the scattered light effects noted earlier. However, there are drawbacks to this type of approach. The step of applying a coating powder or liquid adds cost and time to the tooth contour imaging process. Because the thickness of the coating layer itself has a given thickness and is often non-uniform over the entire tooth surface, measurement errors readily result. No information on relative translucency of the tooth is available when the coating is applied. Further, the applied coating, while it facilitates contour imaging, can tend to mask other problems with the tooth and can thus reduce the overall amount of information that can be obtained. Even where a coating or other type of surface conditioning of the tooth is used, it can still be difficult to provide sufficient amounts of light onto, and to sense light reflected back from, all of the tooth surfaces. Different surfaces of the tooth can be oriented at 90 degrees relative to each other, making it difficult to direct enough light for accurately imaging all parts of the tooth, whether or not a coating is applied.
There have been attempts to adapt structured light surface-profiling techniques to the challenges of tooth structure imaging. For example, U.S. Pat. No. 5,372,502 entitled “Optical Probe and Method for the Three-Dimensional Surveying of Teeth” to Massen et al. describes the use of an LCD matrix to form patterns of stripes for projection onto the tooth surface. Another approach is described in U.S. Patent Application Publication 2007/0086762 entitled “Front End for 3-D Imaging Camera” by O'Keefe et al. U.S. Pat. No. 7,312,924 entitled “Polarizing Multiplexer and Methods for Intra-Oral Scanning” to Trissel describes a method for profiling the tooth surface using triangularization and polarized light, but needing application of a fluorescent coating for operation. Further, U.S. Pat. No. 6,885,464 entitled “3-D Camera for Recording Surface Structures, In Particular for Dental Purposes” to Pfeiffer et al. discloses a dental imaging apparatus using triangularization but also requiring the application of an opaque powder to the tooth surface for imaging.
The use of a powder or other surface coating, which may help facilitate contour imaging, prevents color shade information from being obtained at the same time. Thus, color shade information and surface contour information must be obtained separately, making it difficult to register the shade and shape information to each other.
An approach to obtaining both shape and shade information from one apparatus uses confocal imaging is described in U.S. Pat. No. 7,319,529 entitled “Method and Apparatus for Colour Imaging a Three-Dimensional Structure” to Babayoff. As best understood, a few hundred images of the tooth are taken, at incremental focus distances, and relative pixel intensity is used as a measure of surface contour for multiple points on the tooth surface. Color and depth data thus obtained are then combined in order to obtain and display the shape and color shade of the tooth.
While confocal imaging methods may have some advantages, there are disadvantages to using such methods. Considerable image processing resources can be required with such an approach, depending on the pixel resolution used. When this processing is done externally at a connected host computer or processor, there can be considerable overhead due to the needed volume of data transferred to the host computer or processor for confocal devices. There are also disadvantages related to optical components and design. Optical path requirements differ between what is efficient and beneficial for shape measurement and what is needed to effectively measure tooth color shade. Confocal imaging requires telecentric illumination and imaging paths, thus limiting the field of view of the imaging device.
It can be appreciated that an apparatus and method that provides both accurate surface contour imaging of the tooth and color shade data would help to speed reconstructive dentistry and could help to lower the inherent costs and inconvenience of conventional methods, such as those for obtaining a cast or other surface profile for a crown, implant, or other restorative structure.